“pmset -g assertions” will show you why it thinks it’s awake, it could be a silent video playing in a web browser or something. (and of course, if you can ssh in to run that it must be awake.)
they already mentioned why it was awake: downloading updates.
...which it only does when connected to power so I'm honestly failing to understand what the problem may be. Anyway: Settings -> Software updates -> Advanced -> Download new updates when available -> uncheck.
For everything else: Battery -> Power adaptor -> Wake for network access -> uncheck (may also take care of the above, dunno)
I'm glad I stopped using Apple stuff 10 years ago, their macs are gradually devolving into iDevices.